Photodynamic therapy ("PDT") generally involves administration of a drug (photosensitizer) which, when irradiated with light, becomes electronically activated such that it may interact with molecular oxygen to generate reactive oxygen species. The reactive oxygen species are believed responsible for killing of targeted cells, which are generally those associated with an unwanted hyperproliferating state.
Unless a photosensitizer compound is to be used solely for the treatment of superficial skin diseases, it is important that the wavelength(s) at which the compound absorb light be optimized. Preferably, the compound should absorb strongly in the red region of the spectrum (650-800 nm). Light scattering and the presence of endogenous chromophores (such as hemoglobin) results in very poor penetration of tissues by light at wavelengths below about 600 nm. This means that the large absorption band (the so called Soret band) displayed by porphyrins in the region of 400 nm is not available, practically speaking, for photosensitizer activation in PDT.
Instead, the longer wavelength Q absorption bands must be used. However, the longest wavelength Q band for Photofrin.RTM. (see U.S. Pat. No. 5,059,619), a first generation porphyrin photosensitizer, is only at about 630 nm. Although this wavelength is long enough to permit useful photodynamic therapy approaches for some tumors, it is not ideal. Since light penetration of human tissues typically doubles between 630 and 750 nm, a photosensitizer absorbing at 750 nm would be far more effective at treating thick tumors than, for example, Photofrin.RTM.. However, increasing the absorption wavelength of photosensitizer compounds beyond about 800 nm (into the infrared), would not give rise to further improvements, since the involved energy transitions are insufficient to generate a sufficiently energetic excited state, and corresponding activated oxygen species.
Accordingly, there is a considerable medical need to develop classes of photosensitizers having optimized light absorption properties. One such improved class of photosensitizers are the so called "green monohydrobenzoporphyrins" which are derived from natural porphyrins by Diels-Alder type reactions at one of the functional groups attached to the porphyrin core. An example of such a compound, which is currently in phase III clinical trials, is BPDMA which shows considerable absorption at about 688 nm. In this regard, see for example, U.S. Pat. Nos. 5,095,030; 5,171,749; 5,776,966 and the like. Finally, the porphyrin core structure is characteristically non-polar, and such structures need to be modified by the addition of groups having sufficient polarity to improve the solubility and amphiphilicity properties of the compound, and to improve the rate of metabolism or clearance in the body.
The core structure known itself as porphyrin, as mentioned above, is presented below in comparison with that of chlorin. In principle, the simplest way to increase the wavelength of absorption of a porphyrin would be by reduction of a double bond therein to give the corresponding chlorin (reduction of a porphyrin to a chlorin results in an increase in both the intensity and wavelength of the longest absorption band--providing a shift of about 25 nm and typically a substantial increase in extinction coefficient). ##STR3##
Although reducing agents such as diimide are available that will regioselectively reduce only the double bond targeted for simply conversion of a porphyrin to a chlorin, such reactions are typically reversible. Oxygen in the air is well known to oxidize chlorins back to porphyrins and this impacts not only the synthesis, but also the storage and clinical use of the resultant compound. Accordingly, there are only a limited number of cases where such procedures have been used to synthesize chlorins (see for example R. Bonnet, Chem. Soc. Rev., 1995, p. 19, and R. Bonnet et al., Biochemical Journal, 261, p. 277, 1989, in relation, for example, to the synthesis of tetrakis (m-hydroxyphenyl) chlorin, "m-THPC").
Examples of known chlorins that contain unsaturated exocyclic rings fused to the skeleton between a meso position and its adjacent .beta.-position include purpurins and benzochlorins. Fusion of such exocyclic rings has the advantage of substantially limiting oxidation of the prepared chlorin at the previously reduced pyrrole ring. ##STR4##
A number of purpurins have been described. See, for example, R. B. Woodward et al., J. Am. Chem. Soc., 82, p. 3800, 1960, and also J. H. Fuhrhop et al., Angew. Chem. Int. Ed. Engl., 14, p. 361, 1975 describing an octaethyl purpurin which absorbs at 695 um. A number of purpurins have been shown to have photodynamic activity, and the most effective member of this class of compounds may be a tin etiopurpurin formed by cyclization of meso-[.beta.-(2-ethoxycarbonyl)vinyl] etioporphyrin, followed by metallation with tin chloride. This compound is currently in clinical trials (A. R. Morgan et al., Photochem. Photobiol., 51, p. 589, 1990; A. R. Morgan et al., J. Med. Chem., 32, p. 904, 1989).
The synthesis of a benzochlorin was first described by D.P. Arnold et al. J.C.S. Perkin I, p.1660, 1978, and sulfonation of a benzo ring thereof was effected in concentrated sulfuric acid (see A. R. Morgan et al., Photochem. Photobiol. 55, p.133, 1992 and B. C. Robinson et al., SPIE Optical Methods for Tumor Treatment and Detection: Mechanisms and Techniques in Photodynamic Therapy V, 2675, p. 179, 1996). The resulting sulfonic acid group can be used as a platform for further derivatization, such as to modify bioactivity. An additional pathway to effect derivatization of a benzochlorin involves reaction of a metallated benzochlorin at the meso position adjacent to a gem-diethyl group to yield an iminium salt (D. Skalkos et al., Photochem. Photobiol.,59, p. 175 1994). Photodynamic activity has been demonstrated for various benzochlorins (see A. R. Morgan et al., Photochem. Photobiol. 55, p.133, 1992, and A. R. Morgan et al., Tetrahedron Letters, 35, p. 5347, 1994).
The preparation of a pyridinoporphyrin has been described, C. Alonso et al., Tetrahedron Letters, 38(15), pp. 2757-2758, 1997 wherein the pyrindinyl nitrogen atom, or the pyridinyl ring, may serve as a platform for further derivatization. However, such a compound is believed to lack the optimized absorption profile characteristic of the pharmaceutically useful chlorins.
As aforementioned, the presence of an exocyclic ring fused to the core structure of a chlorin at the site of the reduced pyrrole ring substantially prevents re-oxidation thereof. It would be further advantageous to derivatize the exocyclic ring to optimize biological properties such as solubility, physiological clearance, or to enhance amphiphilicity, that is the presence of both polar and non-polar domains thus enhancing interaction with both polar and non-polar environments. Although derivatization has been described, for example, for benzochlorin by sulfonation under strongly acidic conditions, there is a clear need to develope more flexible methods whereby a large number of such pharmaceutically useful chlorins can be prepared. As described below, the present invention provides such methodology and resultant compounds.